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21. Dissolved Oxygen Sensor Fundamentals
The sensor used, is the amperometric type of Clark Sensor and is
suitable for the measurement of oxygen pressures in the range 0 to 100
cm of mercury. While the probe actually reads partial pressure of
oxygen, the circuit is calibrated to be read in percentage saturation or
parts per million (Milligrams/litre). The operation of probes of the Clark
type relies on the diffusion of oxygen through a suitable membrane into
a constant environment of 0.1 molar potassium chloride. Measurements
are best performed with a reasonable flow past the membrane. At
sufficiently high flow rates, the oxygen current is totally independent of
the flow (a few cm/sec.). The cell must not be shaken however or
unstable readings will result from electrolyte surge bringing new oxygen
from the reservoir to the working cathode surface.
21.1 Operating Principle
The Clark oxygen sensor consists of a gold cathode and a silver/silver
chloride anode, placed in an electrolyte solution. This solution is
contained behind a plastic membrane. In this case the plastic is
0.025mm intermediate density polyethylene sheet. PTFE (Teflon) can be
supplied for special applications. It must be realised that using
membranes of very different thicknesses will result in an error in the
temperature compensation that is applied in the instrument for the
membrane permeability. This coefficient (here +4.2%/
o
C at 25
o
C) is for
this thickness polyethylene. A polarising voltage of about 800 millivolts is
applied between the two electrodes. The gold electrode is placed close
to the membrane and because of the polarising voltage, oxygen diffusing
through the membrane will be reduced at the gold electrode.
Equation:
O
2
+ 2H
+
+ 2 electrons
→
H
2
O
2
This reduction process will produce a current through the oxygen sensor.
A load resistor (actually a thermistor in this case) situated in the sensor
itself, converts this current into a voltage proportional to the oxygen
partial pressure. The thermistor provided within the body of the sensor
can have a temperature coefficient of -4.2%/
o
C. This gives an accurate
temperature compensation for the temperature/permeability effect of the
membrane to oxygen, over a range of
±
20
o
C about a centre value of
25
o
C. Note this compensation is not for the solubility effects. A separate
sensor also included achieves this.